1. Field of the Invention
The present invention relates to a ladder surface acoustic wave filter, to be more specific, a ladder surface acoustic wave filter that has desired transmission characteristics and a reduced size.
2. Description of the Related Art
An existing surface acoustic wave filter has been widely used as a filter for a mobile communication device and an intermediate frequency (IF)-stage filter for a television receiver.
Further, as a surface acoustic wave filter with low loss, which has a large pass band and is excellent in selection characteristics, such a ladder surface acoustic wave filter has been known to constitute a ladder filter circuit by alternately arranging a series arm resonator and a parallel arm resonator between an input terminal and an output terminal.
In the ladder surface acoustic wave filter, various efforts have been made in designing a circuit in order to obtain desired transmission characteristics (frequency attenuation characteristics). For example, in the ladder surface acoustic wave filter disclosed Japanese Unexamined Patent Application Publication No. 2004-173245, an inductor is provided between the parallel arm resonator and the ground so as to shift a resonance point of the parallel arm resonator to the low frequency side and form a second resonance point (sub-resonance point) at the high frequency side relative to an anti-resonance point. This enlarges the pass band of the ladder surface acoustic wave filter and forms an attenuation pole in a stop band at the high frequency side relative to the pass band so as to obtain the desired transmission characteristics.
In the above-mentioned existing ladder surface acoustic wave filter, when an inductance value of the inductor that is provided between the parallel arm resonator and the ground is large, there is a problem that the ladder surface acoustic wave filter is increased in size due to the inductor. In particular, when the sub-resonance point of the parallel arm resonator is desired to be formed at the lower frequency side, the inductance value of the inductor that is provided between the parallel arm resonator and the ground needs to be increased in some cases. The increase in the inductance value of the inductor increases the ladder surface acoustic wave filter in size in some cases.
For example, when the inductor that is provided between the parallel arm resonator and the ground is formed on a mounting substrate or a mounting package, the mounting substrate or the mounting package is increased in size, resulting in a problem that the ladder surface acoustic wave filter is increased in size. To be more specific, when the inductor is formed on the mounting substrate having a configuration in which a plurality of ceramic layers are laminated, an inductor electrode formed of a conductive material is arranged between the ceramic layer and the ceramic layer. When an inductor having a large inductance value is required as the inductor that is provided between the parallel arm resonator and the ground, the inductor electrode needs to be long, so that the mounting substrate needs to be made larger in a planar dimension in order to ensure an arrangement space therefor.
Also in the case where the inductor that is provided between the parallel arm resonator and the ground is prepared as a separated inductor component and is mounted on the mounting substrate or the mounting package together with a piezoelectric substrate on which the series arm resonator and the parallel arm resonator are formed, when the inductor having a large inductance value is required as the inductor that is provided between the parallel arm resonator and the ground, a large inductor component needs to be used and a space needs to be ensured on the mounting substrate or the mounting package in order to mount the large inductor component thereon. This results in a problem that the ladder surface acoustic wave filter is increased in size.